Materials which abrade readily in a controlled fashion are used in a number of applications, including as abradable seals. Contact between a rotating part and a fixed abradable seal causes the abradable material to erode in a configuration which closely mates with and conforms to the moving part at the region of contact. In other words, the moving part wears away a portion of the abradable seal so that the seal takes on a geometry which precisely fits the moving part, i.e., a close clearance gap. This effectively forms a seal having an extremely close tolerance.
One particular application of abradable seals is their use in axial flow gas turbines. The rotating compressor or rotor of an axial flow gas turbine consists of a plurality of blades attached to a shaft which is mounted in a shroud. In operation, the shaft and blades rotate inside the shroud. The inner surface of the turbine shroud, in both the compressor section and the “hot” combustion section of the engine, is most preferably coated with an abradable material. The initial placement of the shaft and blade assembly in the shroud is such that the blade tips are as close as possible to the abradable coating.
As will be appreciated by those skilled in the art, it is important to reduce back flow in axial flow gas turbines to maximize turbine efficiency. This is achieved by minimizing the clearance between the blade tips and the inner wall of the shroud. As the turbine blades rotate, however, they expand somewhat due to centrifugal force. The tips of the rotating blades then contact the abradable material and carve precisely defined grooves in the coating without contacting the shroud itself. These grooves provide the exact clearance necessary to permit the blades to rotate at elevated temperatures and thus provide an essentially custom-fitted seal for the turbine.
In order for the turbine blades to cut grooves in the abradable coating, the material from which the coating is formed must abrade relatively easily without wearing down the blade tips. This requires a careful balance of materials in the coatings. In this environment, an abradable coating must also exhibit good resistance against particle erosion and other degradation at elevated temperatures.
Erosion resistance is necessary to maintain uniform clearances throughout the life of the engine or engine performance characteristics are adversely affected. Conventional commercial turbine engines have exhibited a two percent increase in airflow around blade tips as a result of seal erosion after approximately 3,000 flights. Much of this may be attributed to erosion of the abradable seal and blade airfoil tip, and to rub interactions between the blade tips and the seal. In military engine applications, where gas path velocities are relatively high, erosion resistance is of paramount importance.
There are several air seals used in a compressor section of a gas or aircraft engine. Historically the oldest is feltmetal, which comprises a plurality of metal fibers. Disadvantages of this seal include the fact that it has to be brazed to the substrate material and it is highly porous. A number of other abradable coatings have been proposed, including cellular or porous metallic structures; hard ceramics such as ZrO2 and MgO; a metal matrix of aluminum-silicon with embedded polymer particles; or hexagonal boron nitride powder particles. The disadvantage of these latter coatings is their limited temperature capability at. 315° C. for the polymer coating and 480° C. for the hexagonal boron nitride coating.
Abradable materials used at high temperatures in the compressor section of turbine engines also include NiCrAl/Bentonite coatings and abradables such as the one described in U.S. Pat. No. 5,434,210, which discloses a composite powder for thermal spraying comprising three components, one of metal or ceramic matrix materials, a solid lubricant, and a polymer. Typical as-sprayed coatings comprise a Co alloy matrix with dispersed particles of hexagonal boron nitride and polymer. The polymer is subsequently burned out and the final very porous structure contains only hexagonal boron nitride particles dispersed throughout the Co-based matrix. The coatings prepared from this material have acceptable abradability but low erosion resistance.
The search for suitable materials for use in the compressor section of the turbine is a result of the problem of higher heat levels as stages approach the combustion chamber of the engine. Higher temperatures require higher service temperature materials. Materials that are sensitive to high temperature oxidation such as plastics, graphite or hexagonal boron nitride become fugitive materials above their service temperatures and leave only a weakened skeleton that is susceptible to high erosion or complete degradation and spallation. Other materials such as bentonite-containing material can change hardness and become abrasive at elevated temperatures.
Abradables used in the high temperature combustion section of the turbine have been developed by making thermal barrier coatings (TBC) porous. This has been achieved by incorporation of temperature degradable materials such as high temperature polymers, and/or the use of thermally sprayed hexagonal boron nitride, both of which provide a porous coating. The resulting coating is either heat treated to decompose the degradable material or it is burned out during operation of the turbine. The problem with these materials is that the resulting porous coating lacks mechanical strength, which causes the coatings to fail structurally after periods of thermal cycling. This makes the coating useless for dimensional control and dangerous to the structural integrity of the turbine section due to destruction of the. TBC. There remains a need for thermally stable abradable coatings for use in high temperature environments that provide the desired level of erosion resistance, abradability and thermal stability.